I got Spectrum yesterday and noticed that even though I have History and A&E on my package, and can watch them online, they won't show up on the channel list.

I also had problems with Discovery, Food, Animal Planet, HGTV and TLC all giving the error:
403 You've exceeded the maximum number of streaming sessions. To watch this program, sign out of one of your other sessions, and try again.: stream limit reached

If I manually rescan one by one after waiting a few minutes, it works.

Spectrum has issues with authorizing too many services too quickly, and requires a one- to two-hour wait between retrying the unscanned channels. This is a known issue, but something unique to Spectrum and not really within the developers' hands.

As mentioned above, A+E networks do not provide live feeds. (It was also indicated in the TVE announcement and posting.)

A+E Networks does not have live stream available on there websites. Best thing for anybody wanting this is to use the contact us link on the website like history.com and complain about them not offering a live stream of the channel you subscribe too. If they receive enough complaints they may begin to offer it.

Added support in latest prerelease.
You'll have to update (via click-and-hold on Check for Updates), then after the upgrade: click the pencil icon under your TVE source, click EDIT at the top right, scroll down to 6043 and hit Rescan.